A biosensor (or biological sensor) is an analytical device including a biological element and a transducer that converts a biological response into an electrical signal. Certain biosensors involve a selective biochemical reaction between a specific binding material (e.g., an antibody, a receptor, a ligand, etc.) and a target species (e.g., molecule, protein, DNA, virus, bacteria, etc.), and the product of this highly specific reaction is converted into a measurable quantity by a transducer. Other sensors may utilize a non-specific binding material capable of binding multiple types or classes of molecules or other moieties that may be present in a sample, such as may be useful in chemical sensing applications. The term “functionalization material” may be used herein to generally relate to both specific and non-specific binding materials. Transduction methods used with biosensors may be based on various principles, such as electrochemical, optical, electrical, acoustic, and so on. Among these, acoustic transduction offers a number of potential advantages, such as being real time, label-free, and low cost, as well as exhibiting high sensitivity.
An acoustic wave device employs an acoustic wave that propagates through or on the surface of a piezoelectric material, whereby any changes to the characteristics of the propagation path affect the velocity and/or amplitude of the wave. Presence of functionalization material embodied in a specific binding material along an active region of an acoustic wave device permits a specific analyte to be bound to the functionalization material, thereby altering the mass being vibrated by the acoustic wave and altering the wave propagation characteristics (e.g., velocity, thereby altering resonance frequency). Changes in velocity can be monitored by measuring the frequency, amplitude magnitude, or phase characteristics of the sensor and can be correlated to a physical quantity being measured.
In the case of a piezoelectric crystal resonator, an acoustic wave may embody a bulk acoustic wave (BAW) propagating through the interior (or “bulk”) of a substrate, or a surface acoustic wave (SAW) propagating on the surface of the substrate. SAW devices involve transduction of acoustic waves (commonly including two-dimensional Rayleigh waves) utilizing interdigital transducers along the surface of a piezoelectric material, with the waves being confined to a penetration depth of about one wavelength. In a BAW device, three wave modes can propagate, namely, one longitudinal mode (embodying longitudinal waves, also called compressional/extensional waves, and two shear modes (embodying shear waves, also called transverse waves), with longitudinal and shear modes respectively identifying vibrations where particle motion is parallel to or perpendicular to the direction of wave propagation. The longitudinal mode is characterized by compression and elongation in the direction of the propagation, whereas the shear modes consist of motion perpendicular to the direction of propagation with no local change of volume. Longitudinal and shear modes propagate at different velocities. In practice, these modes are not necessarily pure modes as the particle vibration, or polarization, is neither purely parallel nor purely perpendicular to the propagation direction. The propagation characteristics of the respective modes depend on the material properties and propagation direction respective to the crystal axis orientations. Since shear waves exhibit a very low penetration depth into a liquid, a device with pure or predominant shear modes can operate in liquids without significant radiation losses (in contrast with longitudinal waves, which can be radiated in liquid and exhibit significant propagation losses). The ability to create shear displacements is beneficial for operation of acoustic wave devices with fluids (e.g., liquids) because shear waves do not impart significant energy into fluids.
Certain piezoelectric thin films are capable of exciting both longitudinal and shear mode resonance, such as hexagonal crystal structure piezoelectric materials including (but not limited to) aluminum nitride (AlN) and zinc oxide (ZnO). To excite a wave including a shear mode using a piezoelectric material arranged between electrodes, a polarization axis in a piezoelectric thin film must generally be non-perpendicular to (e.g., tilted relative to) the film plane. In biological sensing applications involving liquid media, the shear component of the resonator is used. In such applications, piezoelectric material may be grown with a c-axis orientation distribution that is non-perpendicular relative to a face of an underlying substrate to enable a BAW resonator structure to exhibit a dominant shear response upon application of an alternating current signal across electrodes thereof. Conversely, a piezoelectric material grown with a c-axis orientation that is perpendicular relative to a face of an underlying substrate will exhibit a dominant longitudinal response upon application of an alternating current signal across electrodes thereof.
Typically, BAW devices are fabricated by micro-electro-mechanical systems (MEMS) fabrication techniques, owing to the need to provide microscale features suitable for facilitating high-frequency operation. In the context of biosensors, functionalization materials (e.g., specific binding materials; also known as bioactive probes or agents) may be deposited on sensor surfaces by microarray spotting (also known as microarray printing) using a microarray spotting needle. Functionalization materials providing non-specific binding utility (e.g., permitting binding of multiple types or species of molecules) may also be used in certain contexts, such as chemical sensing.
Biochemical sensors may incorporate multiple resonators, such as at least one reference region devoid of functionalization material, as well as one or more sensing regions including one or more functionalization materials. It may be desirable to incorporate multiple sensing regions in series or parallel configurations. Presence of multiple resonators (as well as fluidic connections) presents packaging constraints that tend to increase the size and cost of sensor devices, due to the need to provide multiple electrical and fluidic connections. Additionally, conventional biochemical sensing devices may suffer from inconsistent distribution of target species in a sample and/or a low rate of analyte binding that may extend the time necessary to complete measurement of a particular sample.
Accordingly, there is a need for fluidic devices incorporating multiple BAW resonator structures, such as for biosensing or biochemical sensing applications, and that overcome limitations associated with conventional devices.